Ok, ok, so there have been demons before. Not only have we just learned that, but we know they can be defeated by mobians and slimes ten years old or younger, but I think they were at least 9 or 10 because Lime has only been in her complex form during that period of time. This is a castle full of ten year olds. This is going to be a peace of cake. We don't know for sure if this demon is the same kind of demon they've fought, but none of them have said anything about this one being remarkable to them in any way. If anyone kind of understands my thought process, I'm just going to skip over a bit of reasoning and say that Mi was in the right here with his actions.